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Monday, February 16, 2015

Dynamic Serialization of Messages

Call it message marshalling, streaming, or serialization, you'll have to convert from a message data structure to a series of bytes and back, if you want to send a message between two processes. You might cast the message pointer to a byte pointer, take the size and slam the bytes into a the socket, but you won't be able to deal with different cpu architectures (byte ordering), there may be hidden padding between fields, or unwanted fields, there may be tighter representations on the wire than you have in memory. There are many reasons to have some kind of serialization system. The following version has some of the best trade offs I've come across. It was partially inspired by simple json parsers like LitJSON, and partially by a library called MessagePack, which discover the fields of a data structure automatically.

So I kind of hate the boiler plate and manual drudgery of ToStream/FromStream functions added to each message subclass. It always seemed like there should be an automatic way of implementing that code. Google Protocol Buffers, or Thrift and others make you specify your data structures in a separate language, then run a compiler to generate the message classes and serialization code. That always seemed clumsy to me, and was extra work to deal with excluding the generated files from source control, more custom stuff in your maven, makefile, or ms proj files. Plus I always think of the messages as *my* code, not something you generate. These are personal preferences that led to the energy needed to come up with the idea, not necessarily full justification for what resulted. In the end, the continued justification is that it is super easy to maintain, and has a very desirable side effect of fixing a long standing problem of mismatching protocols between client and server. So here's the outline (maybe I'll post the code as part of the Quantum system some day).

We have a large number of message classes, but are lazy, and don't want to write serialization code, and we always had bugs where the server's version of the class didn't match the client's. The server is in Java, and the client is in C#. Maybe the byte order of the client and server are different. Ideally, we could say: Send(m), where m is a pointer to any object, and the message is just sent. Here's how:
- Use introspection (Java calls it Reflection), to determine the most derived class of m, if you have a reflection based serializer constructed for that type, use it, else create one.
- To create one, walk each field of the type, and construct a ReaderWriter instance for that type, appending it to list for the message type. Do this recursively. ReaderWriter classes are created for each atomic type, and can be created for any custom type (like lists, dictionaries, or application classes that you think you can serialize more efficiently). Cache the result so you only do this once. You may want to do this for all message types ahead of time, but that is optional. You could do it on the first call to Send(m)
- As you send each message, find its serializer, and hand the message instance in. The system will walk the list of ReaderWriters and will serialize the whole message. To make this work, the ReaderWriter classes must use reflection to access the field (by reading during send, and by writing during message arrival). This is pretty easy to do in Java, and C#, or interpretted languages like Python, Lua and Ruby. C++ would be a special case where code generation or template and macro tricks would be needed. Or good old fashioned boiler plate. Sigh.
- As a message arrives, you allocate an instance of the type (again, using reflection), then look up the serializer, and fill it in from the byte buffer coming in.

This works fine across languages and architectures. There is a small performance hit in reading or setting the fields using reflection, but it is not too bad, since you only scan the type once at startup, and you keep the accessor classes around so you don't have to recreate anything for each message.

Once you have all this metadata about each message type, it is easy to see how you can make a checksum that will change any time you modify the message class definition. That checksum can be sent when the client first connects to the server. If you connect to an old build, you will get a warning or be disconnected. It can include enough detail that the warning will tell you exactly which class doesn't match, and which field is wrong. You may not want this debugging info in your shipping product (why make it easy for hackers by giving them your protocol description), but the checksums could be retained. The checksum would include the message field names, and types, so any change will trigger a warning. We chose to sort the field serialization alphabetically, and ignored capitalization. That way differences in field order on the client and server didn't matter, and capitalization differences due to language naming conventions were ignored. And atomic types were mapped appropriately.

Another consideration was to delay deserialization as long as possible. That way intermediate processes (like an Edge Server) didn't have to have every message class compiled in. Message buffers could be forwarded as byte buffers without having to pay deserialization/serialization costs. This also allowed deserialization to occur on the target thread of a multi-threaded event handling system.

One necessary code overhead in this system is that the class type of each arriving message has to be pre-registered with the system, otherwise we can't determine which constructor to run with reflection, and we don't know which reflection based serializer to use (or construct, if this is the first use of it since the app started). We need a mapping between the message type identifier in the message header, and the run time type. This registration code allows message types to be compact (an enum, or integer), instead of using the type name as a string (which could be used for reflection lookup, but seemed too much overhead per message send). It has a nice side effect, which is we know every type that is going to be used, so the protocol checking system can make all the checksums ahead of time and verify them when a client makes a connection (instead of waiting for the first instance of each message to detect the mismatch). We might have been able to make use of message handler registration to deduce this message type list, and ignore any messages that arrived that had no handlers.

Some of these features exist in competing libraries, but not all of them were available when we built our system. For example, MessagePack didn't have checksums to validate type matching.